If this is your first visit, be sure to
check out the FAQ by clicking the
link above. You may have to register
before you can post: click the register link above to proceed. To start viewing messages,
select the forum that you want to visit from the selection below.

"Annealing" LEDs to regain lost brightness

I was just reading about how NASA did this to some red GaA LEDs built into a tape recorder on the Galileo spacecraft after the LEDs were damaged by radiation. I wonder if this works for GaN LEDs as well?

Re: "Annealing" LEDs to regain lost brightness

holy cow!

"Galileo's flight team then began a series of "annealing" sessions, where current was passed through the LEDs for hours at a time to heat them to a point where some of the crystalline lattice defects would be shifted back into place, thus increasing the LED's light output. After about 100 hours of annealing and playback cycles, the recorder was able to operate for up to an hour at a time."

I'm amazed that they had the ability to adjust the led drive current like that!

so they would heat the leds up and let them cool slowly, with the goal of letting the atoms that had gotten bumped out of the crystalline structure move back into place??

If your GaN leds have suffered damage to their crystalline structure, it might work. The hard part would be knowing how much heat and how long. I'm betting that the NASA folks spent a lot of time in the lab working out the process before trying it out on the spacecraft.

Re: "Annealing" LEDs to regain lost brightness

Hi, that was an interesting method. Of course, they had little choice but to try something, as the main communications array was down, and the back up array ran so slowly that they were recording data on a 4 track tape for "time shift data telemetry". The LEDs under discussion were the ones for the servo drives in the tape deck, which had failed.

When we use LEDs, you sort of see this effect already. While not as prominent of an effect as it used to be, we used to (5 years ago) burn-in the LEDs with a 24 hour run at full power. This could knock down the Vf as much as 0.5 volts. Modern LEDs are much more consistent, so they already come with the reduced Vf.

The crystal structure that was "repaired" was a GaAs / InGaAs type, which anneals at much lower temperatures than GaN, and those were also a much simpler device structure than what is run today.

Re: "Annealing" LEDs to regain lost brightness

Fascinating. A very similar thing was recently discovered about Flash memory.

Flash memory wears out over time, in large part due to an effect known as charge trapping. Flash retains information by storing electrons on a "floating gate" that is completely insulated. So how do you get the electrons there? By using an extremely high voltage gradient (over a volt per nanometer -- think about that -- that's a gigavolt per meter!) to induce Fowler-Nordheim tunneling of elections into the floating gate. The sheer violence of this process causes a certain amount of stray hot carrier injection into the surrounding oxide, which eventually makes it impossible to read the floating gate charge reliably.

What was just discovered is that by baking old flash memory in an oven, you can dislodge the trapped charges in the oxide. Using this technique, they were able to restore dead flash memory to 80% of its original lifetime. Manufacturers are now seriously investigating putting heating elements into the NAND die stack.

There is one awkward thing about this discovery: normally, the way you do accelerated testing of electronics is by heating them, because that makes everything die faster. Well, now what? How do you test flash memory endurance if heating makes it last longer?

Re: "Annealing" LEDs to regain lost brightness

There are several very interesting posts in here. >1GV/m charge differentials inside FLASH memory? No wonder they wear out. But suddenly it occurs to me that those FLASH SSDs that people put in their laptops nowadays are actually doing themselves a favor by getting so hot when operating, and maybe people shouldn't complain about the heat.

Re: "Annealing" LEDs to regain lost brightness

Originally Posted by fyrstormer

There are several very interesting posts in here. >1GV/m charge differentials inside FLASH memory? No wonder they wear out. But suddenly it occurs to me that those FLASH SSDs that people put in their laptops nowadays are actually doing themselves a favor by getting so hot when operating, and maybe people shouldn't complain about the heat.

Does that mean we get to start seeing chips with heater or filament connections like vacuum tubes have?